Field of the Technology
The present disclosure is directed to methods for producing titanium alloys having high strength and high toughness. The methods according to the present disclosure do not require the multi-step heat treatments used in certain existing titanium alloy production methods.
Description of the Background of the Technology
Titanium alloys typically exhibit a high strength-to-weight ratio, are corrosion resistant, and are resistant to creep at moderately high temperatures. For these reasons, titanium alloys are used in aerospace and aeronautic applications including, for example, critical structural parts such as landing gear members and engine frames. Titanium alloys also are used in jet engines for parts such as rotors, compressor blades, hydraulic system parts, and nacelles.
Pure titanium undergoes an allotropic phase transformation at about 882° C. Below this temperature, titanium adopts a hexagonally close-packed crystal structure, referred to as the α phase. Above this temperature, titanium has a body centered cubic structure, referred to as the β phase. The temperature at which the transformation from the α phase to the β phase takes place is referred to as the beta transus temperature (Tβ). The beta transus temperature is affected by interstitial and substitutional elements and, therefore, is dependent upon impurities and, more importantly, alloying elements.
In titanium alloys, alloying elements are generally classified as α stabilizing elements or β stabilizing elements. Addition of α stabilizing elements (“α stabilizers”) to titanium increases the beta transus temperature. Aluminum, for example, is a substitutional element for titanium and is an α stabilizer. Interstitial alloying elements for titanium that are α stabilizers include, for example, oxygen, nitrogen, and carbon.
Addition of β stabilizing elements to titanium lowers the beta transus temperature. β stabilizing elements can be either β isomorphous elements or β eutectoid elements, depending on the resulting phase diagrams. Examples of β isomorphous alloying elements for titanium are vanadium, molybdenum, and niobium. By alloying with sufficient concentrations of these β isomorphous alloying elements, it is possible to lower the beta transus temperature to room temperature or lower. Examples of β eutectoid alloying elements are chromium and iron. Additionally, other elements, such as, for example, silicon, zirconium, and hafnium, are neutral in the sense that these elements have little effect on the beta transus temperature of titanium and titanium alloys.
FIG. 1A depicts a schematic phase diagram showing the effect of adding an α stabilizer to titanium. As the concentration of α stabilizer increases, the beta transus temperature also increases, which is seen by the positive slope of the beta transus temperature line 10. The beta phase field 12 lies above the beta transus temperature line 10 and is an area of the phase diagram where only β phase is present in the titanium alloy. In FIG. 1A, an alpha-beta phase field 14 lies below the beta transus temperature line 10 and represents an area on the phase diagram where both α phase and β phase (α+β) are present in the titanium alloy. Below the alpha-beta phase field 14 is the alpha phase field 16, where only α phase is present in the titanium alloy.
FIG. 1B depicts a schematic phase diagram showing the effect of adding an isomorphous β stabilizer to titanium. Higher concentrations of β stabilizers reduce the beta transus temperature, as is indicated by the negative slope of the beta transus temperature line 10. Above the beta transus temperature line 10 is the beta phase field 12. An alpha-beta phase field 14 and an alpha phase field 16 also are present in the schematic phase diagram of titanium with isomorphous β stabilizer in FIG. 1B.
FIG. 10 depicts a schematic phase diagram showing the effect of adding a eutectoid β stabilizer to titanium. The phase diagram exhibits a beta phase field 12, a beta transus temperature line 10, an alpha-beta phase field 14, and an alpha phase field 16. In addition, there are two additional two-phase fields in the phase diagram of FIG. 10, which contain either α phase or β phase together with the reaction product of titanium and the eutectoid β stabilizing alloying addition (Z).
Titanium alloys are generally classified according to their chemical composition and their microstructure at room temperature. Commercially pure (CP) titanium and titanium alloys that contain only α stabilizers such as aluminum are considered alpha alloys. These are predominantly single phase alloys consisting essentially of α phase. However, CP titanium and other alpha alloys, after being annealed below the beta transus temperature, generally contain about 2-5 percent by volume of β phase, which is typically stabilized by iron impurities in the alpha titanium alloy. The small volume of β phase is useful in the alloy for controlling the recrystallized α phase grain size.
Near-alpha titanium alloys have a small amount of β phase, usually less than 10 percent by volume, which results in increased room temperature tensile strength and increased creep resistance at use temperatures above 400° C., compared with the alpha alloys. An exemplary near-alpha titanium alloy may contain about 1 weight percent molybdenum.
Alpha/beta (α+β) titanium alloys, such as Ti-6Al-4V (Ti 6-4) alloy and Ti-6Al-2Sn-4Zr-2Mo (Ti 6-2-4-2) alloy, contain both alpha and beta phase and are widely used in the aerospace and aeronautics industries. The microstructure and properties of alpha/beta alloys can be varied through heat treatments and thermomechanical processing.
Stable beta titanium alloys, metastable beta titanium alloys, and near beta titanium alloys, collectively classified as “beta alloys”, contain substantially more β stabilizing elements than alpha/beta alloys. Near-beta titanium alloys, such as, for example, Ti-10V-2Fe-3Al alloy, contain amounts of β stabilizing elements sufficient to maintain an all-β phase structure when water quenched, but not when air quenched. Metastable beta titanium alloys, such as, for example, Ti-15Mo alloy, contain higher levels of β stabilizers and retain an all-β phase structure upon air cooling, but can be aged to precipitate α phase for strengthening. Stable beta titanium alloys, such as, for example, Ti-30Mo alloy, retain an all-β phase microstructure upon cooling, but cannot be aged to precipitate α phase.
It is known that alpha/beta alloys are sensitive to cooling rates when cooled from above the beta transus temperature. Precipitation of α phase at grain boundaries during cooling reduces the toughness of these alloys. Currently, the production of titanium alloys having high strength and high toughness requires the use of a combination of high temperature deformations followed by a complicated multi-step heat treatment that includes carefully controlled heating rates and direct aging. For example, U.S. Patent Application Publication No. 2004/0250932 A1 discloses forming a titanium alloy containing at least 5% molybdenum into a utile shape at a first temperature above the beta transus temperature, or heat treating a titanium alloy at a first temperature above the beta transus temperature followed by controlled cooling at a rate of no more than 5° F. (2.8° C.) per minute to a second temperature below the beta transus temperature. The titanium alloy also may be heat treated at a third temperature.
A temperature-versus-time schematic plot of a typical prior art method for producing tough, high strength titanium alloys is shown in FIG. 2. The method generally includes an elevated temperature deformation step conducted below the beta transus temperature, and a heat treatment step including heating above the beta transus temperature followed by controlled cooling. The prior art thermomechanical processing steps used to produce titanium alloys having both high strength and high toughness are expensive, and currently only a limited number of manufacturers have the capability to conduct these steps. Accordingly, it would be advantageous to provide an improved process for increasing strength and/or toughness of titanium alloys.